Generally, this application relates to gas sensors. In particular, this application relates to gas sensors used to determine the amount of a gas in a fluid.
Gas sensors may be used to measure the amount of one or more gases in a fluid. The term “amount” should be understood to include concentration or partial pressure. To obtain a measurement, a gas sensor may be at least partially immersed in a fluid. The term “fluid” should be understood to include one or more liquids or gases. Exemplary liquids may include mineral oil, water, or alcohols. Exemplary gases may include hydrogen, oxygen, carbon dioxide, carbon monoxide, or acetylene. One example of a gas sensor may be based on the catalytic dissociation of gas on the surface of a palladium alloy. Molecular species such as gaseous hydrogen or other gases may readily dissociate on the surface of the alloy. The protons from the hydrogen may then be free to dissolve into the matrix of the metal. At low levels of the hydrogen, these dissolved protons may migrate to insulating interfaces and may be detected using, for example, capacitors, diodes, or transistors that shift their physical properties in response to the dipole layer formed by the accumulation of the atomic hydrogen at the interface.
As the hydrogen concentration increases, and as the hydrogen becomes more dissolved into the matrix of the palladium alloy, the resistance of the alloy may increase, and may be detected using resistors or transistors. Not only may the dissolved hydrogen provide scattering sites for the free electrons, but it can also distort the lattice and shift the Fermi levels of the electrons. The net effect may be an increase in resistance that is proportional to the amount of dissolved hydrogen and hence it is proportional to the amount of hydrogen gas above the palladium alloy. Since resistance may also have relatively strong temperature dependence, the temperature of the gas sensor has to be controlled and/or measured to obtain a substantially accurate amount measurement. To obtain a substantially accurate measurement, a gas sensor may be set to a substantially uniform and constant temperature. When a gas sensor is operated at a higher temperature, it may have a faster response time but may be less accurate. When a gas sensor is operated at a lower temperature, it may have a slower response time but may be more sensitive and/or accurate.
A heating device may be used to control the temperature of the gas sensor. However, when only a heating device is used to control the temperature of the gas sensor, the gas sensor must operate at a temperature higher than the temperature of the fluid in which it is at least partially immersed, in order to operate the sensor at a substantially uniform and constant temperature. In that circumstance, the operational temperature of the device is limited to a temperature that is higher than the temperature of the fluid in which the gas sensor is at least partially immersed. The result may be to affect the accuracy and/or sensitivity of the gas sensor. Though turning the heater off allows operation at a lower temperature near the temperature of the fluid in which the gas sensor is at least partially immersed, that often results in poor thermal control and negatively affects the accuracy and/or sensitivity.
In certain types of gas sensors, the solubility of the gas, and hence the sensitivity of the gas sensor, may increase at lower operating temperatures. One such system is a hydrogen sensor with palladium alloys. Operating these types of gas sensors at relatively lower temperatures may allow for relatively higher sensitivity. Additionally, certain drift processes in gas sensors may be thermally related and become pronounced at higher operating temperatures. Lower operating temperatures may allow the gas sensor to remain more accurate over a longer period of time.
Conversely, the response times for certain gas sensors to accurately detect the amount of gas may decrease as temperatures increase, such that operating the gas sensor at higher temperatures may be helpful for the gas sensor to respond relatively quickly to changes in the gas being sensed.
It may be desirable to provide a gas sensor that solves these and other problems.